The long-term objectives of this project are to develop an innovative set of probe molecules for targeted diagnosis and therapy of human cancer. The preferred approach is to place an artificial receptor on a target cell, and then to load the receptor with a small molecule that delivers photons for imaging or radiation for therapy. This allows us to use the well-developed target selection available for antibodies, in concert with the rapid pharmacokinetics of small molecules. We add the important feature that the molecules studied here bind with infinite affinity: they selectively form complexes that do not dissociate. This allows maximum time on target, limited only by natural turnover processes. It is a well-known problem in tumor targeting that conventional, reversibly-binding antibodies with high affinity do not penetrate efficiently beyond the surface of a tumor. This is usually called the binding-site barrier, and its basis is the long bound lifetime exhibited by a high-affinity antibody on its target. Weakly binding antibodies, or more usually their monovalent fragments, do not share this problem because they bind and dissociate frequently-but for the same reason, they do not remain in the tumor long enough to be effective. This paradoxical need for ligands of simultaneous low and high affinity significantly limits the efficacy of antibodies for imaging and above all for therapy of solid tumors. Selective binding without dissociation solves an important practical problem in probe capture for tumor targeting, but may also present an obstacle to efficacious distribution of the ligand throughout the tumor. A more subtle alternative, which should lead to far more effective tumor penetration of antitumor antibodies or engineered fragments, is to combine weak with infinite binding. If we begin with low-affinity antitumor antibodies and prepare constructs that can bind permanently, but only after many association-dissociation events, this should allow the constructs not only to permeate the tumor but also to eventually attach permanently to their targets.